When communicating data over a network, overhead information is required for reliable delivery from the source device to the destination device. Examples of overhead information that may be required include an address of the source device and an address of the destination device. Error checking and error correcting information may also be sent. These types of overhead information are generally sent as part of a packet or frame of data. Other types of overhead information may be used to ensure information is reliably routed between a source and a recipient. An example of such overhead may be an acknowledgement packet.
Regardless of how the overhead information is reflected in ongoing communication, it is known that aggregating information to reduce the number of interactions required to convey data from a source to a destination can increase the ratio of information bits to overhead bits conveyed. Increasing this ratio makes for more efficient network communication.
Aggregation schemes have been developed to take advantage of more efficient communication. For network protocols that require data to be sent in packets, aggregation may be achieved by aggregating the data from multiple packets. FIGS. 1A-1D illustrate known schemes for packet aggregation.
FIG. 1A illustrates conceptually a frame 120, formatted for wireless communication. In this example, the frame contains a Media Access Control (MAC) Service Data Unit (MSDU) according to TCP/IP or the “Ethernet” protocol with additional fields added to support wireless communication. To the MSDU may be added a media access control (MAC) header, forming what is essentially an Ethernet frame. A radio preamble 101, radio header 103 and frame check sequence (FCS) 107 may be added to “wrap” the Ethernet frame for wireless communication.
In frame 120, media access control (MAC) header 105 as well as radio preamble 101, radio header 103 and frame check sequence (FCS) 107 constitute overhead for transmission of MSDU 110. Further, in the example illustrated, an acknowledgment (ACK) frame 130 is returned from the receiving device when frame 120 is received successfully. ACK frame 130, which contains a radio preamble 101, radio header 103 and ACK payload 109, is also overhead. The time between sending of frame 120 and receipt of ACK frame 130 may also be regarded as overhead. Other time that may be included in the overhead is the time to ensure a channel is available for transmission of the frame.
The ratio of data to overhead may be increased by increasing the amount of data transmitted in a frame. One approach to increasing the amount of data is illustrated in FIG. 1B. In FIG. 1B, MSDU 111 takes up a larger percentage of the frame 140 than is the case of MSDU 110 in frame 120.
An alternative approach to reducing overhead is sometimes called frame aggregation. In frame aggregation, data from multiple frames may be combined into a single communication, which is communicated with less overhead than if the multiple frames were communicated separately.
In networks, such as 802.11n wireless networks, protocols for aggregating frames have been defined. IEEE 802.11n supports aggregation technologies including Media Access Control (MAC) Service Data Units (MSDU) aggregation and MAC Protocol Data Unit (MPDU) aggregation. These aggregation techniques are supported at the MAC layer, and some manufacturers of chipsets for implementing wireless network interfaces have incorporated functionality to support MSDU or MPDU aggregation.
FIG. 1C illustrates an example of MSDU aggregation. MSDU aggregation allows the information from what otherwise would be transmitted as multiple frames to be aggregated into a single frame. Specifically, data units 112-1 through 112-N, which represent data that might otherwise each be sent as separate frames, are aggregated into a frame 150 which can be sent in a single communication. A single radio preamble 101, radio header 103 and frame check sequence (FCS) 107 is sent for frame 150. In MSDU aggregation, the data is destined to the same end point, which is identified by a single MAC header 108. Likewise, a single ACK frame 130 (FIG. 1A) may be sent in response to the entire MSDU frame 150, avoiding the need to wait for separate acknowledgements for transmission of separate packets. Thus, the entire frame is sent with overhead similar to that of a single frame, but with much more data.
MPDU is similar. However, the individual MSDU's may be destined for separate endpoints, though the entire frame is sent to one wireless access point. As illustrated by frame 160 in FIG. 1D, each MSDU 110-1 to 110-N has a corresponding MAC header 105-1 to 105-N, which identifies the end point for that data. This collection of data units is wrapped with a radio preamble 101, radio header 103 and frame check sequence (FCS) 107. Because a separate MAC header 105-1 to 105-N is included for each MSDU, an access point receiving frame 160 can forward each MSDU to a different destination. Though, the separate MAC headers 105-1 to 105-N constitute more overhead information than in MSDU frame 150.